IN_GETIFA(9) NetBSD Kernel Developer's Manual IN_GETIFA(9)
NAMEin_getifa -- Look up the IPv4 source address best matching an IPv4 desti-
nation
SYNOPSISoptions IPSELSRC#include <netinet/in_selsrc.h>struct ifaddr *in_getifa(struct ifaddr *ifa, const struct sockaddr *dst0);
DESCRIPTIONin_getifa enforces the IPv4 source-address selection policy. Add the
source-address selection policy mechanism to your kernel with optionsIPSELSRC. options IPSELSRC lets the operator set the policy for choosing
the source address of any socket bound to the ``wildcard'' address,
INADDR_ANY. Note that the policy is applied after the kernel makes its
forwarding decision, thereby choosing the output interface; in other
words, this mechanism does not affect whether or not NetBSD is a ``strong
ES''.
An operator affects the source-address selection using sysctl(8) and
ifconfig(8). Operators set policies with sysctl(8). Some policies con-
sider the ``preference number'' of an address. An operator may set pref-
erence numbers for each address with ifconfig(8).
A source-address policy is a priority-ordered list of source-address
ranking functions. A ranking function maps its arguments, (sourceaddress, source index, source preference, destination address), to inte-
gers. The source index is the position of source address in the inter-
face address list; the index of the first address is 0. The sourcepreference is the preference number the operator assigned to sourceaddress. The destination address is the socket peer / packet destina-
tion.
Presently, there are four ranking functions to choose from:
index ranks by source index; lower indices are ranked more
highly.
preference ranks by source preference; higher preference numbers
are ranked more highly.
common-prefix-len ranks each source address by the length of the longest
prefix it has in common with destination address;
longer common prefixes rank more highly.
same-category determines the "categories" of source and destinationaddress. A category is one of private, link-local, or
other. If the categories exactly match, same-category
assigns a rank of 2. Some sources are ranked 1 by
category: a link-local source with a private destina-
tion, a private source with a link-local destination,
and a private source with an other destination rank 1.
All other sources rank 0.
Categories are defined as follows.
private RFC1918 networks, 192.168/16, 172.16/12,
and 10/8
link-local 169.254/16, 224/24
other all other networks---i.e., not private,
not link-local
To apply a policy, the kernel applies all ranking functions in the policy
to every source address, producing a vector of ranks for each source.
The kernel sorts the sources in descending, lexicographical order by
their rank-vector, and chooses the highest-ranking (first) source. The
kernel breaks ties by choosing the source with the least source index.
The operator may set a policy on individual interfaces. The operator may
also set a global policy that applies to all interfaces whose policy he
does not set individually.
Here is the sysctl tree for the policy at system startup:
net.inet.ip.selectsrc.default = index
net.inet.ip.interfaces.ath0.selectsrc =
net.inet.ip.interfaces.sip0.selectsrc =
net.inet.ip.interfaces.sip1.selectsrc =
net.inet.ip.interfaces.lo0.selectsrc =
net.inet.ip.interfaces.pflog0.selectsrc =
The policy on every interface is the ``empty'' policy, so the default
policy applies. The default policy, index, is the ``historical'' policy
in NetBSD.
The operator may override the default policy on ath0,
# sysctl -w net.inet.ip.interfaces.ath0.selectsrc=same-category,common-prefix-len,preference
yielding this policy:
net.inet.ip.selectsrc.default = index
net.inet.ip.interfaces.ath0.selectsrc = same-category,common-prefix-len,preference
The operator may set a new default,
# sysctl -w net.inet.ip.selectsrc.debug=> same-category,common-prefix-len,preference
# sysctl -w net.inet.ip.interfaces.ath0.selectsrc=
yielding this policy:
net.inet.ip.selectsrc.default = same-category,common-prefix-len,preference
net.inet.ip.interfaces.ath0.selectsrc =
In a number of applications, the policy above will usually pick suitable
source addresses if ath0 is configured in this way:
# ifconfig ath0 inet 64.198.255.1/24
# ifconfig ath0 inet 10.0.0.1/24
# ifconfig ath0 inet 169.254.1.1/24
# ifconfig ath0 inet 192.168.49.1/24 preference 5
# ifconfig ath0 inet 192.168.37.1/24 preference 9
A sysctl, net.inet.ip.selectsrc.debug, turns on and off debug messages
concerned with source selection. You may set it to 0 (no messages) or 1.
SEE ALSOifconfig(8), sysctl(8)STANDARDS
The family of IPv6 source-address selection policies defined by RFC3484
resembles the family of IPv4 policies that in_getifa enforces.
AUTHORS
David Young <dyoung@NetBSD.org>
BUGS
With options IPSELSRC, a new interface ioctl(2), SIOCSIFADDRPREF, was
introduced. It ought to be documented in inet(4). Also, options(4)
ought to cross-reference this manual page.
This work should be used to set IPv6 source-address selection policies,
especially the family of policies defined by RFC3484.
NetBSD 6.0.1 February 22, 2007 NetBSD 6.0.1

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